NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System) commonly referred to as the Global Positioning System (GPS) was developed by the United States Department of Defense. GPS uses satellites and ground equipment to determine position anywhere on Earth. At least 24 satellites are operational at all times. Unlike paying an Internet Service Provider to access the Internet, anyone with a receiver can use the system at no cost.

The GPS system consists of three components; ground stations, a constellation of satellites in Earth orbit, and receivers.

Satellite Constellation

Each GPS satellite transmits data that indicates its location and the current time. For accurate position determination by receivers each satellite must have an extremely accurate time reference which is provided by an onboard atomic clock.

The data signals transmitted by the constellation of GPS satellites are synchronized as it is the difference in arrival time of signals (equates to different distances from each satellite) from different satellites that is used by GPS receivers to determine their position.

Receivers

Using their own onboard time reference, receivers can determine the travel times of signals received from different satellites. Knowing the speed of the signal (speed of light) and the travel time from a particular satellite allows the distance to that satellite to be determined.

As each satellite transmits its exact position in its broadcast signals, a receiver can determine its distance from a number of reference points (satellites). Using a mathematical principle called trilateration a GPS receiver (GPSr) can use calculated distances from four GPS satellites to determine its position in three dimensions (longitude, latitude and altitude).

Just as the GPS satellites require an accurate time reference (onboard atomic clock) so do GPS receivers. Cost and practicality rule out atomic clocks for receivers. The accurate time dilemma is overcome by receivers using the incoming signals from four or more satellites to determine their own inaccuracy and the correct reference time to atomic clock accuracy.

Ground Stations

The NAVSTAR GPS satellite constellation is managed by the United States 50th Space Wing. Ground stations precisely track each satellite and provides the satellites with regular navigational updates.

GPS Accuracy

Position accuracy is determined by the receiver.

Less expensive, general purpose GPS receivers have an accuracy of about 10-20 meters (33 - 66 feet).

Wide Area Augmentation System (WAAS) enabled GPS receivers can determine their position to within 2 meters (6 feet).

Receivers employing Differential GPS (DGPS) can determine their position to the order of 10mm (less than half an inch). DGPS requires additional stationary receivers for corrective positioning information.

NASA GPS Introduction Video

This NASA produced short film (3:30 min) describes the Global Positioning System.

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